1. Field of the Invention
The present invention relates to an improved galvanizing method and apparatus incorporating the use of a fluidized bed of zinc powder for galvanizing of elongated metal substrates. More particularly, such metal substrates are galvanized by first electro-statically depositing zinc powder on the metal substrate followed by liquefying, reflowing and cooling the zinc coating within an inert atmosphere to obtain a coating thickness that is uniform and selectable over a wide range.
2. Description of the Prior Art
It has been known in the metal industry to apply a zinc coating on a base steel product by immersion into a bath of molten zinc, the purpose being to increase the life of the product by covering the base material with a protected coating metal. Under present day practices, the coating of zinc by itself or in combination with another metal may be applied by either a hot dip method or by an electroplating method.
In the hot galvanizing process, the stock to be galvanized is first annealed and then cleaned in a pickling bath of hydrochloric acid. After rinsing in hot water, the stock is immediately passed through a bath of flux, such as zinc chloride, or sal ammoniac, the purpose of which is to prevent rusting of the stock when it is dried, and to aid the reaction between zinc and iron. The stock is now ready for the application of molten zinc, or "spelter," as it is called. This is contained in a spelter pan of such size as is required for the galvanizing operation undertaken. The pan is supported on a brick setting and directly fired from below. The stock is dipped in the molten zinc for sufficient time to permit the zinc to alloy with the surface of the iron so that a coating of zinc will remain on the stock when it is withdrawn from the spelter pan. The excess zinc is allowed to drip from the galvanized stock, or is mechanically wiped from it. Air knives may be used to control the thickness of zinc before solidifying it. The galvanizing operation is now complete except for cooling the stock and cutting it into sheets, wrapping it on reels, or otherwise preparing it for commerce.
Electro-galvanizing is carried out in a plating vat, usually made of wood. In one portion of this vat are bus wires, to which the positive of the plating circuit is connected. The vat is filled with an electrolyte of some zinc salt. Zinc sulfate is frequently used. The stock to be plated is suspended in the electrolyte by metallic support which are connected to the negative of the plating circuit. Current is supplied from a special plating generator, and the circuit is completed through the electrolyte, decomposing it and plating the zinc on the cathodes, which are the pieces being galvanized. Preparations prior to electro-galvanizing are similar to those in hot galvanizing.
In the hot dip galvanizing process, while less costly, a substantial lead time is necessary so as to make available the necessary tonnage of liquid zinc to form the bath in which the strip or plate is immersed. For example, it requires several hours to melt zinc bars to provide the appropriate zinc bath. The coating thickness of the zinc is not accurately controllable although the use of air knives has achieved good results with coatings thickness above G20. Air knives, however, are generally ineffective with coating thicknesses between G20 and G10 and electro-galvanizing methods are too costly.
Accordingly, it is an object of the present invention to provide a method and apparatus to galvanize a ferrous substrate using a coating material comprised of finely divided powder adhered to the substrate by an electrostatic charge before galvanizing passage to a reflow station.
It is a further object of the present invention to provide a galvanizing process and apparatus in which a potential of an electrostatic charge is selected according to a desired coating thickness determined by a thickness of adhered powder galvanizing material to a substrate.
It is yet a further object of the present invention to provide a method and apparatus for galvanizing a ferrous substrate in which the thickness of the galvanized material can be selected to provide coating thickness of between G20 and G10.